The invention relates to a metalliferous storage material and a method of producing the material. It is first pointed out that, under the term metalliferous material, atomic metals, metal alloys, intermetallic phases of metals or compound materials as well as corresponding hybrids are to be understood.
It is known that, on the basis of reversible metal hydrides, hydrogen storage devices, so-called hydride storage devices, can be formed. The storage device can be charged while heat is released, that is, hydrogen is bound by chemo-sorption and discharged by the application of heat. Hydrogen storage devices can therefore be excellent energy storage devices for mobile and/or stationary applications. They might form in the future a notable storage potential since no noxious emissions are released during the discharge of the hydrogen storage devices.
Very suitable for such hydride storage devices are the so-called nano-crystalline hydrides, which are capable of rapidly storing and releasing the hydrogen. Their manufacture however has been very expensive, so far. Their manufacture, so far, has involved high-energy grinding of elemental components or pre-alloys of nano-crystalline alloys, wherein the grinding procedure can be very long. In a final process step, these nano-crystalline alloys were subjected, depending on the conditions, to a multistage heat treatment under a high hydrogen pressure to be hydrogenated thereby. For many alloys, furthermore, a multiple charging and discharging with hydrogen is necessary to achieve full capacity.
Alternatively, it has been tried to synthesize the respective hydrides by grinding in a hydrogen atmosphere or in a pure chemical way. It has been found, however, that, in this way, the yield of the desired hydrides is smaller and partially additional undesirable phases occur.
Furthermore, certain phases could, or respectively can, not be formed with the known conventional methods.
The German patent application No. 197 58 384.6 discloses a method for the manufacture of nano-crystalline metal hydrides with which the manufacture of stable and meta-stable hydrides or hydride-meta-stable alloys is possible with a very high yield of up to 100%. The method described in the mentioned German patent application can be performed with easily controllable limiting conditions and with a relatively small energy consumption.
In order for such a hydrogen storage device to rapidly provide the energy stored therein when needed and to permit rapid charging of the hydrogen storage device, it is desirable that the reaction speed during hydrating and dehydrating of metals at low temperatures is kept very high that is a very high reaction speed is to be aimed at.
To this end, so for, the reaction surface has been increased by reducing the size of the particles/crystals of the materials to be hydrogenated or dehydrogenated as far as this was technically feasible. Other means for increasing the reaction speed included the addition of nickel, platinum or palladium.
The disadvantage of the measures known so far for increasing the reaction speed during the hydrogenation and particularly the dehydrogenation, that is, the delivery of the hydrogen from the hydrogen storage device is that the available speeds are in-sufficient for hydrogen storage devices intended for technical applications.
It is therefore the object of the present application to provide a metalliferous material, such as a metal, a metal alloy or an intermetallic phase, compound materials of metals as well as corresponding hydrides with which, during hydrogenation and dehydrogenation, the reaction speeds are so high, that they are technically feasible for use as energy storage devices. A method is to be provided by which the manufacture of a metalliferous material such as a metal, a metal alloy, an intermetallic phase or a compound material of the materials or corresponding hydrides can be performed in a simple and inexpensive way such that metals manufactured in this way can be used commercially as hydrogen storage devices in a cost-effective manner and with the technically necessary high reaction speed during hydrogenation and dehydrogenation.
In a metalliferous storage material for hydrogen a metal oxide is provided in or on the surface of the metalliferous material as a catalyst for the hydrogenation or dehydrogenation of the metalliferous storage material.
In accordance with the invention, the fact is utilized that, in comparison with pure metals, metal oxides are brittle, whereby a smaller particle size and a homogeneous distribution of the metal oxide in the material is achieved. As a result, the reaction kinetics are substantially increased in comparison with metallic catalysts. Another advantage is that the metal oxides are available as catalysts generally at much lower prices than metals or respectively, metal alloys so that also the aim of commercial utilization at reasonable costs for the metalliferous materials according to the invention can be achieved.
Basically, the metal oxide is an oxide of atomic metals such as the oxide of the metals Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Ce, Mo, Sn, La, Hf, Ta, W. In accordance with an advantageous embodiment of the invention, the metal oxide may also consist of mixed oxides of the metals, particularly of the metals listed earlier or of mixtures of the metal oxides.
Advantageously, the metal oxide or metal oxides may be formed by rare earth metals or metal oxides or mixtures of rare earth metals.
In an advantageous embodiment of the invention, the metal has a nano-crystalline structure, wherein, equally advantageously, also the catalyst has a nano-crystalline structure. If the metal and/or the catalyst have a crystalline structure, the reaction surface and, consequently, the reaction speed of the hydrogenation or, respectively, the dehydrogenation of the metalliferous material are increased.
The method according to the invention for the manufacture of such a metalliferous material is characterized in that the metalliferous material and/or the catalyst are subjected to a mechanical grinding procedure with the object to form, from both components, a powder with an optimized reaction surface of the metalliferous material as well as a uniform distribution of the catalyst.
The grinding procedure itself may be selected, depending on the metalliferous material and/or the catalyst, to be differently long so as to achieve the optimal desired reaction surface and an optimal distribution of the catalyst of the metalliferous material according to the invention. In this connection, it may be advantageous if the metalliferous material as such is first subjected to the grinding and the catalyst is added subsequently to the grinding process, however the process may be reversed, that is, the catalyst may be first subjected to the grinding followed by the metalliferous material. Also, these distinguished possible procedures for the grinding are selected depending on the metalliferous materials and depending on the catalyst to be added.
In order to prevent reactions with the ambient gas during the grinding of the metaliferous material (metal, metal alloy, intermetallic phase, compound material as well as the hydrides thereof) the method is preferably performed under an inert atmosphere wherein the inert gas is preferably argon.
As already mentioned, the duration of the grinding process for a metalliferous material (metal, metal alloy, intermetallic phase, compound material as well as the hydrides thereof) and the catalyst is variably selectable depending on the metalliferous material and the selected catalyst. Preferably, the duration of the grinding process is in the area of 1 to 200 hours.
In another type of the method for the manufacture of a metalliferous material, which may be used as electrode material at least for secondary elements, at least one metal oxide is formed on the surface of the electrode material in situ by contact with oxygen from elements of the electrode/material or by direct supply of oxygen. In this way, a catalyzing oxide can be formed in situ from elements of the hydride storage material.
Preferably, during performance of the method, the surface of the electrode material is activated chemically and/or mechanically before the oxide is formed, whereby the oxide formation of the metal can be improved.
The invention will now be described in detail with reference to various diagrams, which describe the hydrogenation and dehydrogenation behavior as well as other important parameters.